The present invention relates to communications on heterogeneous networks, cable and wireless, and more particularly to the dynamic configuration of the network points of interconnection for transmission error management.
The impairments and losses of information that arise during the transfer of data over a network may give rise to significant effects in the processing of data on the client side. Errors in the transmission of multimedia data may for example give rise to strong visual distortions on the presentation of the multimedia content transmitted.
The use of suitable error management mechanisms has proved necessary to maintain a certain quality of service, all the more so since the particular characteristics of wireless networks have shown the limits of existing mechanisms for cable networks based on static models such as the mechanisms called FEC (Forward Error Correction). This is because the static mechanisms based on the use of information redundancy in data packets prove to be all the more effective, the greater the redundancy introduced. Their use can therefore give rise to significant “overhead” and bandwidth reservation, although insufficient to correct errors generated by a wireless network and constant and therefore unsuited to the fluctuating conditions of wireless networks.
The dynamic error management mechanisms based on retransmission of the ARQ (Automatic Repeat reQuest) or Hybrid ARQ type, although potentially giving rise to unnecessary retransmissions, prove from this point of view to be more suitable.
However, even though these methods have in this context good results, they do have the drawback of causing the element applying the mechanism to ignore the erroneous data that may have not been fully corrected. However, some data, even erroneous, in particular for video streams, may prove useful to the client, in particular to decoders for which the even partial decoding of data make it possible to limit the distortions due to transmission error.
In a context of continuous and real-time transfer (streaming) of a multimedia stream, the streams pass in the form of packets from the server to the client. A deadline is associated with each packet: the packet must be transmitted in compliance with a maximum transmission time. In a system for continuous transfer in real time over heterogeneous networks, each packet transits over cable and wireless networks, before being finally routed to the client. The points of interconnection in the networks, referred to as PICs, receive the packets from one network and retransmit them over another. They thereby constitute major factors in the management of errors generated by the networks by implementing error management mechanisms (for example of the FEC, ARQ or hybrid ARQ type).
However, though the methods conventionally used at PICs for managing errors on wireless networks, of the ARQ or hybrid ARQ type, have good results, they do not make it possible to transmit to the client the erroneous data that it has not been possible to correct fully.
In addition, while the variations in the wireless network are frequent and have an influence on the nature and quantity of transmission errors, these mechanisms do not meet the requirement to be able to adapt the error management policy to the fluctuating transmission conditions and to the nature of the data transmitted.
It is therefore necessary to have available a mechanism for increasing the probability of distribution of significant data in its entirety and on time, by adapting to the variable conditions of the network the error management policy that the PIC must implement using known error detection and correction methods (for example FEC, ARQ and Hybrid ARQ) as a function of various criteria, defined dynamically from information on the networks and the errors observed and inherent in the nature of the data being managed.
The patent U.S. Pat. No. 5,457,701 describes a method for indicating to the client that an error that cannot be corrected by the network has occurred during the transfer of the data via heterogeneous networks in a multipoint communication system.
This system allows the detection of “irreparable” errors that cannot be corrected by the means available at the network. Bits are added by the server to each packet before it is transmitted, so as to reserve information space for error check. These bits are initialized by the server to their default value indicating the integrity of the data.
Thus, when during the transfer of the packet to the client an “irreparable” error is detected by a network point of interconnection, the bits provided for this purpose are modified for a predefined value indicating the impairment of the data.
The erroneous packet thus modified is then transmitted to the client in the original data stream. The client must, on receiving the data, examine the bits reserved for the error check in order to determine the integrity of the data received.
However, this mechanism is based on an identical error management policy for all the packets transferred, whatever their significance in the stream and the constraints relating to them (for example small latency time and high rate).
Moreover, this invention only partially exploits the error correction phase, implemented following the error detection, when it does not lead to the entire correction of the erroneous data. The mechanism described simply makes it possible to indicate that the error detected has not been able to be fully corrected, the data being subsequently transmitted to the client as they stand.
Finally, in this invention, the irreparable data identified by the value of the bits reserved for the error check are transmitted to the client in their original stream with the non-erroneous data, obliging the client to examine all the packets received in order to determine the appropriate processing. Such an invention therefore requires proprietary systems for the clients and servers.
The article entitled “Link-level partial checksum for real-time video transmission over 802.11 Wireless networks” by E. Masala, M. Bottero and J C De Martin (IEIIT-CNR di Torino, Vehicular Technology Conference, 2005, VTC 2005-Spring, 2005 IEEE 61st) describes a technique of developing the error detection process of the IEEE 802.11 standard in order to ensure better quality of the video stream exchanges over wireless networks, by enabling certain packets partially corrupted during their transfer over the network to be simply relayed by the PIC instead of being abandoned.
The study presented in this article concerns particularly the aptness of performing an error detection on all the data bits transferred. It reveals that the application of an error detection mechanism limited to the most pertinent data ensures better quality of data transfer in real time, the data of less importance being relayed to the client as they stand without error check.
The mechanism proposed that stems from this is based on the use of a partial check (checksum) allowing selective dynamic coverage of the data exchanged. When the PIC detects an error in the data area covered by the check, it implements a standard error correction mechanism of the ARQ type. If the area covered by the check cannot be entirely corrected during this phase, the packet is then abandoned. In the contrary case, the packet corrected on the area covered is transmitted to the client whatever the state of the non-covered data.
This mechanism is based on the change in the coverage of the data for the error detection.